yepo, you can easily replace the battery, just go to any watch seller and ask for the CR2032 battery and replace it with your current one, simple as that.

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computer stores sell those too(standard bios battery)

and besides, it's not possible to get a pokemon stuck in a pokewalker.
from what i've read the pokemon is stored in persistent memory(like the memory in a USB stick) and only progress from while it was in the pokewalker is erased.
and it is possible to retrieve the pokemon without the pokewalker near your DS, but you'll get it back exactly the way it was before you sent it to the pokewalker.